A typical EGR system of an engine includes one or more EGR valves for controlling the flow of engine exhaust gas from the engine's exhaust system to the engine's intake system to meter an appropriate amount of exhaust gas into fresh air passing through the intake system where the air supports combustion of fuel in the engine's cylinders. The metered exhaust gas in effect dilutes the air so that in-cylinder temperature rise resulting from combustion is limited from that which would occur in the absence of such dilution. As a consequence, the quantity of oxides of nitrogen (NOx) in the exhaust gas that results from combustion is also limited.
Some EGR systems, especially those designed for compression ignition (i.e. diesel) engines, have one or more heat exchangers for cooling recirculated exhaust gas. Cooling of the exhaust gas can further limit the generation of NOx.
It is recognized in the industry that cooling of recirculated exhaust gas creates the potential for condensation of certain gaseous constituents of the exhaust gas. Over time such condensates may accumulate sufficiently to have a detrimental effect on performance and/or components. For example, coolant passageways in coolers may become restricted, components may corrode, and moving parts may stick.
Condensation may be more extreme and/or perhaps even unavoidable at certain times, such as when a cold-soaked engine is warming after having been started and portions of its EGR system have not yet reached operating temperature. When condensation occurs along an EGR flow path and temperature of surrounding parts is sufficiently low, condensate may freeze and consequently restrict, or even block, the flow until the parts warm sufficiently to thaw the frozen condensate.
To address such situations, known practices in EGR control strategies include delaying and/or limiting EGR under cold-start conditions. Although such measures may be helpful in slowing the accumulation of condensates as the engine ages, they do impact the quantity of NOx in tailpipe emissions.